Fixes a bad escape.

\; caused the following compilation error

  ! Missing $ inserted.
  <inserted text>
                $
  l.7845 }

I have removed the backslash, and placed the semicolon between a pair
of $ to make it render like the one in the grammer-environment.

Change-Id: If57e61f61b460d159b6f14d70c9c9f343114cf70
Reviewed-on: https://dart-review.googlesource.com/71760
Commit-Queue: Daniel Hillerström <hillerstrom@google.com>
Reviewed-by: Erik Ernst <eernst@google.com>

docs/language/dartLangSpec.tex

@@ -7840,7 +7840,7 @@ begin with a \{ character.
 The expression of an expression statement is not allowed to begin with a \{.
 \commentary{
 This means that if some source text could otherwise be parsed as an expression
-followed by a \;, then this grammar production does not apply
+followed by a {\rm `$;$'}, then this grammar production does not apply
 when the expression starts with a \{.
 }
 \rationale{